[RichardP]

I can't make my mind up whether or not it's working properly. What's the best way to test it? Just shine a torch into the sensor with the car with it's lights on and see?

If you do that while it's dark, you should see a significant change in the mirror if it's working.

 

[RichardP]

Unless you have reason to think that the electrical connections are suspect, it's more likely that the dimming properties of the mirror have failed. Mine failed not long ago, starting from the outside with the dimming part gradually getting smaller and smaller in the middle of the mirror.